The present invention relates to a method of manufacturing a prosthesis cuff to receive an amputation stump, for example for a lower-leg prosthesis, and to a rough cuff for use in performing the method. The invention also relates to a prosthesis cuff manufactured by the method.
There is an acute need in the field of orthopedics for an efficient method of manufacturing individually fitted prostheses. Methods employed hitherto are still mainly manual, especially in the case of prosthesis cuffs intended for amputation stumps for example for lower-leg prostheses
It is well known that certain plastics have thermoplastic memory properties, so that for exmaple, if such a plastics material is stretched under certain conditions it will retain its stretched condition until heated to a particular temperature, whereupon it will shrink towards its original form unless physically restrained from doing so. The use of heat-shrinkable rough cuffs having such thermoplastic memory properties has been proposed for the manufacture of the type of prosthesis cuffs under discussion here. According to such prior proposals, such a prefabricated pre-stretched cuff is applied directly around the amputation stump in question and is externally heated to achieve the required fit of the cuff to the outer contour of the amputation stump.
However, in practice it has proved impossible by the proposed methods to achieve the extremely careful fit required between cuff and amputation stump. Thus, for example, in the case of a leg prosthesis, to be able to withstand the considerable forces arising during walking movement of the patient, without damage to the stump, certain delicate points must take up more of the load. This cannot, hwoever, be achieved with the cuffs produced in accordance with such prior proposals, which frequently had a poor fit leading to chafing and pressure sores. It was also found that the shrinkage properties and strength properties together were so limited in such plastic cuffs that any practical use of such directly shrinkable cuffs was unthinkable.
According to established design principles for rough cuffs of this type for leg prostheses, a stump of large diameter generally indicates a heavy, sturdily-built person and the appropriate plastic cuff should therefore be made relatively stronger by making the wall thickness of the cuff relatively thick. Correspondingly, for a stump of lesser diameter, the cuff wall should be made thinner since it may be assumed that such a stump belongs to a slimmer and thus a lighter person. The diameter of an adult person's stump varies by a factor of approximately two and variations in the weight of an adult person are also of this magnitude. This would mean that a cuff of the largest diameter should be designed with a wall thickness approximately twice that of a cuff of the smallest diameter.
If only the shrinkage properties of the plastic are to be utilised, it is found if one starts with a standard rough cuff of given diameter and wall thickness and allows such cuffs to shape themselves to the respective prosthesis stumps by shrinkage, the wall thickness of the finished cuffs will increase as the diameter decreases. This is exactly contrary to the design principles set out above. In order to embrace the above variations in stump sizes by a reasonable margin, a considerable number of standard cuffs is required, with correspondingly large costs for tool-making and stocking.
Attempts have also been made earlier to use a conventional plaster positive of the acutal amputation stump and to perform the final shaping of the cuff around the plaster positive instead of applying prefabricated rough cuffs directly on the amputation stump. This procedure has also proved unsuccessful in practice since the wall material of the cuff is unable during the shrinkage process to follow recesses, hollows and extra load-absorbing depressions normally occurring in the outer surface of the plaster positive. The result is a poor fit, which also gives rise to the drawbacks mentioned above, chafing and pressure sores for the patient.
Finally endeavours have also been made to give the rough cuff its final shape solely by vacuum shaping the cuff around a plaster positive. However, in this process folds occur during the vacuum shaping due to too much excess material in the large cuffs. It will be understood that folds and edges in a prosthesis cuff can in no way be tolerated by the stump for which the cuff is intended.